It is known in the prior art that stitch process information such as bight and work feed instruction can be stored in a static memory such as, for example, a read only memory (ROM).
When a key for selecting a pattern stitch is pressed, a code corresponding to the selected key can be generated by the encoder connected with said key switch and a first address of the stitch pattern is retrieved from the static memory and set in an address counter for use by the address memory unit. In response to synchronous timing pulses, the data for position of bight and work feed of the stitch pattern are fed sequentially from the memory to drive the bight and work feed mechanism. For selecting one of multiple stitch patterns, a multiple selection key on the indicating panel and a shifting switch on said panel can be combined into a composite encoder system and a digital "1" and "0" respectively can be generated as the composite code. Such a system, however, is rather complicated.
Another method of selecting different stitching patterns is by the use of a pressure difference of touch on multiple additional keys. However, the number of keys that can be utilized for selecting such a stitch pattern is restricted considering the limited space available in a sewing machine.
Also considered in the prior art was a method of synchronizing the signal for needle movement with respect to stitch performance with timing of the stitch pattern information. To accomplish this synchronization, an integrated circuit combining a Hall effect element with a multi-stage amplifier has been proposed. In that system, the Hall effect element is placed opposite a permanent magnet. Between the Hall effect element and the magnet, a disc having a magnetic shielding portion is revolved with the main shaft of the sewing machine. As a result, a potential difference is developed at the Hall IC output as the magnetic element is alternately shielded and unshielded. The potential difference can be used as a synchronizing pulse.
However, there are defects involved in the use of this method. First, there are so many different stitch patterns and large amounts of control data to be stored in the memory unit and such small volume and limited space available with a home-use sewing machine that a special integrated circuit element must be developed. Accordingly, compact integration of an exclusive use memory and the use of a control unit is inevitable which causes production costs to be fairly high. Moreover, it is required to change machine types frequently and, in such case, since the stitch pattern signals change, the integrated circuit must change. Thus, this method is not favorable for home-use sewing machines. On the other hand, if a stitch pattern selection device utilizes a combined panel key and selection switch, the mechanical structure is so complicated as to create problems. It may cause error selection and also it is inconvenient because a two-step operation is necessary for a required pattern selection.
To select different stitch patterns by pressure differences of key touch, such as light touch and strong press, it is not only inconvenient to operate, but also requires a complicated pressing mechanism which induces errors in operation.
When a Hall effect device is used for generating a synchronous signal, the Hall effect element acts as a current sensing element. This element always carries such a large current in comparison to the magnetic resistance element which is utilized in the present invention that is caused inferior temperature characteristics and may possibly cause erroneous operation when the hot temperature occurs. Further, the output voltage of a Hall element is so low that it is necessary to add an amplifier circuit. Accordingly, it is compulsory to use an integrated IC with the Hall element which is expensive and thus undesirable.